An industrial pulverizer which is more or less a forerunner of the type for which the present invention is an improvement is described in U.S. Pat. No. 4,264,041 to Kitto and Kowalski. This patent cites four earlier patents--2,275,595, 2,378,681, 2,473,514, and 2,545,254, all relating particularly to the configuration of the throat of the pulverizer. The '041 patent to Kitto and Kowalski itself is directed to improvements in the throat design. It is clear that throat design is of very high importance in the pulverizer art. The throat is the part of the pulverizer which is responsible for ejecting the pulverized coal from the grinding area into a forced air conveyor which will conduct it to a combustion zone.
In U.S. Pat. No. 4,874,135, Provost introduced the concept of a throat ring with specially designed air channels to ensure that the powdered coal is thrust centrifugally as well as upwardly towards the main stream headed to the combustion zone. Thereafter Provost described a wear ring for use with the throat ring, to further minimize wear. The fact that the entire throat ring and wear ring were removable was emphasized in this patent. They were designed so that they could be placed in existing pulverizers with a minimum of labor.
Rotating throats were introduced to the art a few years after the above mentioned Kitto and Kowalski '041 patent. See U.S. Pat. Nos. 4,687,145, 4,721,258, and 4,907,751. Rotating throats were directed to the same problems that faced the art in previous years--primarily, excessive or uneven erosion of various surfaces in the pulverizer, and imperfect distribution of the ground coal or other material. The art is also concerned with minimizing the consumption of power used for the forced air flow, and the unwanted accumulation of coal powder in areas under possibly incendiary or even explosive conditions.
A recent U.S. Pat. No. to Henning et al, 5,340,041, illustrates the continuing practice of maintaining the ledge cover as a fixture on the pulverizer housing when a rotating throat is used.
Replacing a stationary throat with a rotating throat and/or a rotating vane wheel in an existing more or less conventional pulverizer is an expensive and laborious process. The most difficult problem is that the stationary lower throat casting must be destroyed by dismembering it, typically with a plasma torch or by air-arcing. One is then thoroughly committed economically to a rotating throat; any thought of returning to a stationary throat will find the cost prohibitive. The grinding table or yoke must also be tediously machined in place to accommodate attaching the rotating throat segments. Because of the cramped quarters, accuracy of the machining cannot be guaranteed or even, perhaps, expected.
A primary objective of the present invention is therefore to provide a rotatable throat which can be relatively easily put in service on existing pulverizers, but which can also serve as a stationary throat if so desired. At the same time, the design answers the other needs of the art respecting "dribble" of the larger particles, uneven wear of various surfaces in the machine, and optimum power consumption in the air turbine.
While my invention is not limited to application in the Babcock & Wilcox MPS-89 pulverizer, it will be described with particular reference to this machine because it typifies the pulverizers in wide use in the United States and elsewhere, and the design is typical of the pulverizers in which my invention will find use. The B&W MPS-89 employs a rotating grinding table with wear-resistant cast iron track ring and three stationary wheels and tires to pulverize coal for combustion in utility boilers. A good description of this machine may be found on page 9-7 of "Steam"a book published by Babcock & Wilcox Company, copyrighted in 1978; the description is incorporated herein by reference. The wheels and tires are loaded by means of springs compressed by tensioning rods. Hot, temperature-controlled primary combustion air is introduced into an annular space in the pulverizer below the grinding table, and is distributed around the periphery of the grinding table through air ports. The peripheral area including the ports is the throat.
Primary air performs four functions in the pulverizer: drying of the coal in the pulverizer, maintaining a fluidized bed of coal, which circulates coal into the path of the grinding elements, transporting the pulverized coal particles from the fluidized bed into the classifier assembly, where large particles are separated for return to the grinding elements, and transporting suitably-pulverized coal particles out of the classifier to the burners.
The air ports in the pulverizer throat are configured to agitate the fluidized bed vigorously, thereby enhancing the drying action, to redirect and move the fluidized bed of coal back into the path of the grinding elements, and to establish a swirl inside the pulverizer that enhances the cyclonic separation of heavier particles of coal in the classifier.
The throat assembly of the B&W MPS-89 is made up of a stationary lower ring, replaceable upper ring segments, and ledge cover segments which are invariably stationary, fixed to the inside of the pulverizer housing, all arranged desirably to minimize turbulence, which tends to accelerate wear from the abrasive particles. Often, restrictor bars must be welded into the throat openings to adjust the amount of primary air flowing through the ports. Without proper aerodynamics, the restrictor bars aggravate the unwanted turbulence, and accelerate wear of the upper throat segments. Often, there is a mis-match between upper and lower segment ports which traps rejects in the port, leading to clogging of the ports, particularly in areas of low air flow.
As originally designed, the throat assembly is stationary, with an air seal between the rotating grinding table and the throat to minimize air leakage. A stationary throat allows varying pressure differentials (with corresponding variable air flows) along the circumference of the grind ring, which further upsets the proper aerodynamics of the fluidized bed, causes accelerated wear in some areas of the pulverizer, and results in less than optimum performance of the pulverizer with respect to coal fineness at the burner pipes. A number of rotating throat designs have been offered to correct some or all of the problems associated with stationary throats, but replacement of the original throat with any of the rotating style throats widely offered is very costly, labor intensive, and leaves no other options.